Fix tracking of unknown syscalls for 'truss -c'.

This is done by changing get_syscall() to either lookup the known syscall
or add it into the list with the default handlers for printing.

This also simplifies some code to not have to check if the syscall variable
is set or NULL.

Reviewed by: jhb
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D3792