shared: add vectorized parse_field() (#7056)

By vectorizing parse_field() the chain of parse_field() calls in
output_short() can be replaced with a single call receiving a description
of the desired fields and their targets.

While at it, eliminate the repeated strlen() calls performed on constant
field names by making parse_field() receive the field length, and storing
it in the ParseFieldVec at compile time.

Also sort the output_short() fields so the short ones are tried first, for
a minor efficiency gain.

In addition to making the code less repetitive, gcc in my tests now inlines
the parse_fieldv() call in output_short().