Allow "unlimited" abbreviations

Allow "unlimited" abbreviations

Currently we can abbreviate "on/off/enable/disable/yes/no" in boolean
settings,

  (gdb) set non-stop of
  (gdb) set non-stop en

we can abbreviate the items of enumeration commands,

  (gdb) set print frame-arguments scal
  (gdb) set scheduler-locking rep

but we cannot abbreviate "unlimited" in integer commands.

 (gdb) set print elements u
 No symbol "u" in current context.

This commit fixes that.

Testcases will be in gdb.base/settings.exp and gdb.base/options.exp,
in following patches.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* cli/cli-setshow.c (is_unlimited_literal): Allow abbreviations.