Set GDB version number to 7.7.91.

gdb/ChangeLog:

	* version.in: Set GDB version number to 7.7.91.
Document the GDB 7.7.91 release in gdb/ChangeLog

gdb/ChangeLog:

	GDB 7.7.91 released.
Bump GDB version number to 7.7.91.DATE-cvs.

gdb/ChangeLog:

	* version.in: Set GDB version number to 7.7.91.DATE-cvs.
Automatic date update in version.in