Teach functions.sh to sanitize user inputs for ingress and egress target

The codel family of qdiscs and pie allow to specify a latency target.
Implement get_target() to sanitize the user inputs. only accept well
formed specifiers as 5ms or 50usecs. The allowed units were taken
from tc_util.c get_time(), the function that handles the input for tc,
 information was extracted from iproute2 v3.10.0. Misformed target requests
are silently dropped, empty values for target will instruct the qdisc
to use its default value.