syslogd(8) may not always be running before dhcpcd so any startup errors are

syslogd(8) may not always be running before dhcpcd so any startup errors are
lost. Also, it's output with LOG_PERROR is ugly and LOG_PERROR isn't portable.

logger is a new function to solve the above which is identical to syslog(3)
but takes a dhcpcd_ctx for context so it functions in the RTEMS OS.
When writing to stdout/stderr or a given logfile, logger will convert %m
into the error string based on errno, just like syslog(3) does.
If your libc supports %m via printf, then define HAVE_PRINTF_M to remove
a fair chunk of code. There is no way of working out if your libc supports
this, especially in cross-compiled environment.
If the logfile is rotated, send dhcpcd a SIGUSR2 and the logfile will be
re-opened.

This does add about an exta 4K on 64-bit platforms so for size constraints
this can be compiled out by defining USE_LOGFILE to 0.