libcdb-cli: add --offline-only, refactor unstrip and add fetch parser for download libc-database

[the-soloist]

1. make unstrip as default behavior, except file parser
2. add return_raw for search_by_symbol_offsets
3. add fetch arg parser to download and update libc-database

[peace-maker]

Removing this causes cached libcs to not be unstripped even when unstrip was set.

[the-soloist]

It will unstrip in search_by_hash (called by search_by_build_id)

pwntools/pwnlib/libcdb.py

Lines 283 to 285 in b08f4b7

	# Try to find debug info for this libc.
	if unstrip:
	unstrip_libc(cache)

[peace-maker]

It won't if the file was downloaded before and is cached without debug symbols.

pwntools/pwnlib/libcdb.py

Lines 255 to 257 in b08f4b7

	cache, cache_valid = _check_elf_cache('libcdb', search_target, search_type)
	if cache_valid:
	return cache

[the-soloist]

Sorry, I overlooked that. But why not unstrip before return cache?

[peace-maker]

True, that's the better place to put this!