libusermode: extract common logic in PF and injection mode #1385

Job	Run time
init (ubuntu-20.04)	24s
init (ubuntu-latest)	21s
Code QL Analyze (ubuntu-20.04)	5m 38s
compile (ubuntu-20.04)	1m 44s
Code QL Analyze (ubuntu-latest)	3m 35s
scan-build	3m 43s
cognitive-complexity	29s
compile (ubuntu-20.04, -Dbuildtype=debug -Db_lto=false)	1m 13s
Build using autoconf tarball (ubuntu-20.04)	2m 19s
Build using autoconf tarball (ubuntu-latest)	2m 24s
compile (ubuntu-20.04, -Dbuildtype=debug -Db_lto=false -Dplugin-syscalls=false)	1m 9s
compile (ubuntu-20.04, -Dbuildtype=debug -Db_lto=false -Db_sanitize=address,undefined)	1m 17s
compile (ubuntu-20.04, -Dbuildtype=debug -Db_lto=false -Drepl=true)	1m 16s
compile (ubuntu-20.04, -Dbuildtype=debug -Db_lto=false -Dthreadsafety=true)	1m 15s
compile (ubuntu-latest)	1m 30s
compile (ubuntu-latest, -Dbuildtype=debug -Db_lto=false)	1m 19s
compile (ubuntu-latest, -Dbuildtype=debug -Db_lto=false -Dplugin-syscalls=false)	1m 9s
compile (ubuntu-latest, -Dbuildtype=debug -Db_lto=false -Db_sanitize=address,undefined)	1m 23s
compile (ubuntu-latest, -Dbuildtype=debug -Db_lto=false -Drepl=true)	1m 41s
compile (ubuntu-latest, -Dbuildtype=debug -Db_lto=false -Dthreadsafety=true)	1m 7s
	34m 56s

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