fping: retain privileges until after privileged setsockopt

fping: retain privileges until after privileged setsockopt

On Linux, one needs privileges to setsockopt(SO_BINDTODEVICE), and the
current code would drop setuid root privileges too soon.

Temporarily drop privileges instead, and raise them back to issue the
privileged setsockopt().  Once we know we won't need to do any further
privileged setsockopt(), permanently drop privileges.

For now, assume SO_BINDTODEVICE is the only setsockopt that needs this.