llkd: enhance list properties

Because of the limited length of properties, and to ease the
complexity of product and vendor adjustments, the comma separated
list properties will use a leading comma to preserve the defaults
and add or subtract entries with + and - prefixes respectively.

Without the leading comma, the list is explicitly specified as before.

Cleanup:
- use empty() instead of space() == 0 (or converse if != 0)
- if (unlikely) pprocp can not be allocated, to a to_string(ppid) check

For testing, observe before and after llkd_unit_test below to
confirm leading comma effects for example:

livelock: ro.llk.stack=wait_on_page_bit_killable,bit_wait_io,\
                       __get_user_pages,cma_alloc
livelock: ro.llk.stack=...,SyS_openat,...

Test: llkd_unit_test
Bug: 120983740
Change-Id: Ia3d164c2fdac5295a474c6c1294a34e4ae9d0b61