Fix #975, #977: Change scripts from /bin/bash to /bin/sh

Fix #975, #977: Change scripts from /bin/bash to /bin/sh

https://github.com/BestImageViewer/geeqie/pull/975/commits/01d45343f9de35ac6cdf696864066b9b1a5015a0
https://github.com/BestImageViewer/geeqie/pull/977/commits/c84ccd8408a6b88483e6690574390c7b27b49c01

- Change all /bin/bash to /bin/sh
- Make POSIX compliant
- Standardize mktemp calls
- Change some echo calls to printf
- Change some @brief headers
- Update CODING.md for above changes