Browser should pretty-print current directory

[pdmef]

The directory browser does not pretty-print directories entered interactively in the status bar:

• When in '/foo' and entering '../bar', it shows '/foo/../bar'
• When in '/baz' and entering '/foo/../bar', it shows '/foo/../bar'.

The directory shown seems to be correct, so it's a display issue only.

Fixing is not technically hard but I'm not sure where the best place to do so is.

[brendan]

What do you think about doing it in mutt_pretty_mailbox?

[pdmef]

(In [8b783c916893]) Use realpath() in mutt_pretty_mailbox() for paths that may need it. This requires to add the buffer size as parameter since the result may be longer than the original but still fit in the buffer. Closes #2948.