Remove a few duplicate implementations of prefix/suffix comparison
functions, and rename them to starts_with and ends_with.

* cc/starts-n-ends-with:
  replace {pre,suf}fixcmp() with {starts,ends}_with()
  strbuf: introduce starts_with() and ends_with()
  builtin/remote: remove postfixcmp() and use suffixcmp() instead
  environment: normalize use of prefixcmp() by removing " != 0"