nfnl_osf: Improve error handling

nfnl_osf: Improve error handling

For some error cases, no log message was created - hence apart from the
return code there was no indication of failing execution.

If a line load fails, don't abort but continue with the remaining
file contents. The current pf.os file in this repository serves as
proof-of-concept:

Lines 700 and 701: Duplicates of lines 698 and 699 because 'W*' and 'W0'
parse into the same data.

Line 704: Duplicate of line 702 because apart from 'W*' and 'W0', only
the first three fields on right-hand side are sent to the kernel.

When loading, these dups are ignored (they would bounce if NLM_F_EXCL
was given). Upon deletion, they cause ENOENT response from kernel. In
order to align duplicate-tolerance in both modes, just ignore that
ENOENT.

Signed-off-by: Phil Sutter <phil@nwl.cc>