add f2c_string in libFMS.F90

[mlee03]

Description
This PR adds fms_string_utils_f2c_string that is made available via use libFMS

Fixes #1600

How Has This Been Tested?
fms_string_utils_f2c_string can be called in an external module with use libFMS

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes