feature test macros: make _GNU_SOURCE enable everything

previously, a few BSD features were enabled only by _BSD_SOURCE, not
by _GNU_SOURCE. since _BSD_SOURCE is default in the absence of other
feature test macros, this made adding _GNU_SOURCE to a project not a
purely additive feature test macro; it actually caused some features
to be suppressed.

most of the changes made by this patch actually bring musl in closer
alignment with the glibc behavior for _GNU_SOURCE. the only exceptions
are the added visibility of functions like strlcpy which were BSD-only
due to being disliked/rejected by glibc maintainers. here, I feel the
consistency of having _GNU_SOURCE mean "everything", and especially
the property of it being purely additive, are more valuable than
hiding functions which glibc does not have.

include/stdio.h

@@ -174,9 +174,6 @@ int ferror_unlocked(FILE *);
 int fileno_unlocked(FILE *);
 int getw(FILE *);
 int putw(int, FILE *);
-#endif
-
-#ifdef _BSD_SOURCE
 char *fgetln(FILE *, size_t *);
 #endif