Avoid grave accent quoting in stderr diagnostics

Avoid grave accent quoting in stderr diagnostics

A few Emacs diagnostics go directly to stderr, and so can't easily
contain curved quotes (as non-UTF-8 locales might mishandle them).
Instead of bothering to add support for this rarity, reword the
diagnostics so that they don't use grave accent to quote.
* src/alloc.c (mark_memory): Fix comment.
* src/buffer.c (init_buffer):
* src/dispnew.c (init_display):
* src/emacs.c (main, sort_args):
* src/lread.c (dir_warning):
* src/term.c (init_tty):
* src/unexmacosx.c (unexec):
* src/xfns.c (select_visual):
* src/xterm.c (cvt_string_to_pixel, x_io_error_quitter):
Reword stderr diagnostics to avoid quoting `like this'.
* src/unexmacosx.c: Include errno.h.
* src/xfns.c (select_visual): Encode value for locale.