Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Fix more style(9)-related issues (copyright header, spaces after function
names, unnecessary casts)
- Change type of boolean variable from char to bool

Suggested by: jhb, zont, jmallett
Reviewed by: cognet
Approved by: cognet

- Remove underscores from the internal structure name, as it doesn't collide
with the user's namespace.

- Correct size and position variables type from long to size_t.

- Do not set errno to ENOMEM on malloc failure, as malloc already does so.

- Implement the concept of "buffer data length", which mandates what SEEK_END
refers to and the allowed extent for a read.

- Use NULL as read-callback if the buffer is opened in write-only mode.
Conversely, use NULL as write-callback when opened in read-only mode.

- Implement the handling of the ``b'' character in the mode argument. A binary
buffer differs from a text buffer (default mode if ``b'' is omitted) in that
NULL bytes are never appended to writes and that the "buffer data length"
equals to the size of the buffer.

- Remove shall from the man page. Use indicative instead. Also, specify that
the ``b'' flag does not conform with POSIX but is supported by glibc.

- Update the regression test so that the ``b'' functionality and the "buffer
data length" concepts are tested.

- Minor style(9) corrections.

Suggested by: jilles
Reviewed by: cognet
Approved by: cognet

Add fmemopen(3), an interface to get a FILE * from a buffer in memory, along
with the respective regression test.
See http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmemopen.html

Reviewed by: cognet
Approved by: cognet