Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

getlocalbase: Make default path actually configurable

We include paths.h in getlocalbase.c, which defines _PATH_LOCALBASE.
This will override a definition on the command line, meaning it is
impossible to override like how the manpage says you can, and it means
the code to provide a fallback default is dead as the macro is always
defined.

Instead, introduce a new LOCALBASE_PATH macro like LOCALBASE_CTL_LEN
that can be set on the command line and will default to the system's
existing _PATH_LOCALBASE to avoid duplicating the default here.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40975

getlocalbase.3: Fix AUTHORS formatting

MFC after: 1 week

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Change getlocalbase() to not allocate any heap memory

After the commit of the current version, Scott Long pointed out, that an
attacker might be able to cause a use-after-free access if this function
returned the value of the sysctl variable "user.localbase" by freeing
the allocated memory without the cached address being cleared in the
library function.

To resolve this issue, I have proposed the originally suggested version
with a statically allocated buffer in a review (D27370). There was no
feedback on this review and after waiting for more than 2 weeks, the
potential security issue is fixed by this commit. (There was no security
risk in practice, since none of the programs converted to use this
function attempted to free the buffer. The address could only have
pointed into the heap if user.localbase was set to a non-default value,
into r/o data or the environment, else.)

This version uses a static buffer of size LOCALBASE_CTL_LEN, which
defaults to MAXPATHLEN. This does not increase the memory footprint
of the library at this time, since its data segment grows from less
than 7 KB to less than 8 KB, i.e. it will get two 4 KB pages on typical
architectures, anyway.

Compiling with LOCALBASE_CTL_LEN defined as 0 will remove the code
that accesses the sysctl variable, values between 1 and MAXPATHLEN-1
will limit the maximum size of the prefix. When built with such a
value and if too large a value has been configured in user.localbase,
the value defined as ILLEGAL_PREFIX will be returned to cause any
file operations on that result to fail. (Default value is "/dev/null/",
the review contained "/\177", but I assume that "/dev/null" exists and
can not be accessed as a directory. Any other string that can be assumed
not be a valid path prefix could be used.)

I do suggest to use LOCALBASE_CTL_LEN to size the in-kernel buffer for
the user.localbase variable, too. Doing this would guarantee that the
result always fit into the buffer in this library function (unless run
on a kernel built with a different buffer size.)

The function always returns a valid string, and only in case it is built
with a small static buffer and run on a system with too large a value in
user.localbase, the ILLEGAL_PREFIX will be returned, effectively causing
the created path to be non-existent.

Differential Revision: https://reviews.freebsd.org/D27370

Add function getlocalbase() to libutil.

This function returns the path to the local software base directory, by
default "/usr/local" (or the value of _PATH_LOCALBASE in include/paths.h
when building the world).

The value returned can be overridden by 2 methods:

- the LOCALBASE environment variable (ignored by SUID programs)
- else a non-default user.localbase sysctl value

Reviewed by: hps (earlier version)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27236

Revert the whole getlocalbase() set of changes while a different design is
hashed out.

Add the library function getlocalbase and its manual page. This helps to
unify the retrieval of the various ways that the local software base directory,
typically "/usr/local", is expressed in the system.

Reviewed by: se
Differential Revision: https://reviews.freebsd.org/D27022