gdb: Add command 'maint set/show debuginfod download-sections'

This setting controls whether GDB may attempt to download ELF/DWARF
sections from debuginfod servers.

This setting is enabled by default if gdb is built with debuginfod
section download support (requires libdebuginfod 0.188).

Also update debuginfod command help text and gdb.texinfo with
information on section downloading and the new command.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>