#25 davmail.sh should not depend on bash

[Uffe Jakobsen]

davmail.sh contains a shebang depending on /bin/bash
The locations of bash and any other shell may vary on systems and hence bash may not live in /bin/bash
Also currently there is nothing in the davmail.sh shell script construction that is dependant on special features found in bash alone - making it depend on the more standard (bourne) shell (sh) is the most portable approach.

The attached patch changes the shell to (bourne) shell and aligns with the recommendations found in the link below:
https://en.wikipedia.org/wiki/Shebang_%28Unix%29#Portability

[Uffe Jakobsen]

[Mickael Guessant]

Current version does not depend on bash