fopen.3: Explain BSD vs glibc "a+" difference

fopen.3: Explain BSD vs glibc "a+" difference

Where is the initial read position for an "a+" stream?

POSIX leaves this unspecified. Most BSD man pages are silent, and MacOS
has the ambiguous "The stream is positioned at the end of the file", not
differentiating between reads and writes other than to say that fseek(3)
does not affect writes. glibc's documentation explicitly specifies that
the initial read position is the beginning of the file.

My new wording is based on the BSD implementations, so you may prefer
to replace the non-glibc section with "unspecified", or indeed remove
all claims about the initial read position.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>