regexp search for more things

[David Bremner]

This series combines the mid: searching patch left out of 0.24 [1]
with the WIP patch of [2] adding regexp search for tags and paths.  It
is rebased on the regexp fixes of [3], which should hopefully be
merged soon.

[1]: id:20170227023422.26929-5-david at tethera.net
[2]: id:20170308024035.6149-1-david at tethera.net
[3]: id:20170318030303.17344-1-david at tethera.net