Read $GDBHISTSIZE instead of $HISTSIZE

Read $GDBHISTSIZE instead of $HISTSIZE

The HISTSIZE environment variable is generally expected to be read by
shells, not by applications.  Some distros for example globally export
HISTSIZE in /etc/profile -- with the intention that it only affects
shells -- and by doing so it renders useless GDB's own mechanism for
setting the history size via .gdbinit.  Also, annoyances may arise when
HISTSIZE is not interpreted the same way by the shell and by GDB, e.g.
PR gdb/16999.  That can always be fixed on a shell-by-shell basis but it
may be impossible to be consistent with the behavior of all shells at
once.  Finally it just makes sense to not confound shell environment
variables with application environment variables.

gdb/ChangeLog:

	* NEWS: Mention that GDBHISTSIZE is read instead of HISTSIZE.
	* top.c (init_history): Read from GDBHISTSIZE instead of
	HISTSIZE.
	(init_main): Refer to GDBHISTSIZE instead of HISTSIZE.

gdb/doc/ChangeLog:

	* gdb.texinfo (Command History): Replace occurrences of HISTSIZE
	with GDBHISTSIZE.

gdb/testsuite/ChangeLog:

	* gdb.base/gdbinit-history.exp: Replace occurrences of HISTSIZE
	with GDBHISTSIZE.
	* gdb.base/readline.exp: Likewise.