Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Fix warnings during bootstrap on Linux systems

Most warnings are currently off for the boostrap phase, but once D27598
lands they will be enabled again.

Add missing FreeBSD functions to -legacy when building on macOS/Linux

In most cases this simply builds the file from lib/libc for missing
functions (e.g. strlcpy on Linux etc.). In cases where this is not possible
I've added an implementation to tools/build/cross-build.

The fgetln.c/fgetwln.c/closefrom.c compatibility code was obtained from
https://gitlab.freedesktop.org/libbsd/libbsd, but I'm not sure it makes
sense to import it into to contrib just for these three bootstrap files.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D25978