Fix Kpathsea search path configuration in asyprocess.

Fix Kpathsea search path configuration in asyprocess.

From the Kpathsea manual:
  http://www.chemie.fu-berlin.de/chemnet/use/info/kpathsea/kpathsea_4.html

"
A search path can come from many sources. In priority order (meaning
Kpathsea will use whichever it finds first):

   1. A user-set environment variable, e.g., `TEXINPUTS'.
   2. A program-specific configuration file, e.g., an `S /a:/b' line
   in Dvips' `config.ps'.
   3. A line in a Kpathsea configuration file `texmf.cnf', e.g.,
   `TEXINPUTS=/c:/d'. See section below.
   4. The compile-time default (specified in `kpathsea/paths.h').

...

If the highest-priority search path (in the list in the previous
section) contains an extra colon (i.e., leading, trailing, or
doubled), Kpathsea inserts the next-highest-priority search path that
is set at that point.

...

For example, given
  setenv TEXINPUTS /home/karl:
and a `TEXINPUTS' value from `texmf.cnf' of
  .:$TEXMF//tex
then the final value used for searching will be:
  /home/karl:.:$TEXMF//tex