build: define inline'd symbols in src/text-options.c

Avoids 'undefined reference' errors when compiling with coverage
instrumentation.
build: add scripts to generate coverage information
src/column-headers.c: improve error message
tests: add tests, improve coverage
src/field-ops.c: refactor free() code.
src/field-ops.c: document unreachable code.
build: remove unused module 'strnumcmp'
coverage tests: exclude unreachable line
tests: tests sort-pipe failure
gitignore: ignore coverage files
Travis-CI: check coverage after successful build