Added a user tag to option structs

This can be used to further classify options.
Parser functions and option structs have been adapted accordingly.
Moved general options in separate function. Distinction of options by tag.
Split parse_flow_option into two separate functions.

parse_flow_option parses options without endpoint.
parse_flow_option_endpoint parses those that have an endpoint string.
[Code Style] Modified struct names in fg_argparser to comply with rest of project. Added doxygen to fg_argparser files.
[Code Style] Added doxygen comments
Merge branch 'controller_add_tag' into next
Merge pull request #115 from erzb5e6f65/next

Introduce a user tag in argparser option struct
No need to specify EXTRA_DIST

We build a tarball (flowgrind release) not by calling 'make dist'. No
need to specify EXTRA_DIST
Use 'maintainer-clean' rule for deleting files

While building flowgrind tarball delete generated files w/ 'make
maintainer-clean' instead of list them all explicitly
Use .PHONY instead of FORCE target

For rules without recipes or prerequisites, GNU recommends to use
.PHONY, which is more explicit and more efficient. See
https://www.gnu.org/software/make/manual/html_node/Force-Targets.html
Use AC_PATH_TOOL instead of AC_PATH_PROG

Needed for cross-compiling
Makefile rule for doxygen

- add a autoconf check for doxygen
- introduce new make rule for doxygen (make html)
[Coding Style] Force line breaks in Makefile.am
[Coding Style] increase readability of configure.ac

- force line breaks
- group command logically together
- add comments to group of commands
- fix indention
- use squared brackets for function parameters
Use AC_SUBST instead of mangle LDFLAGS and CFLAGS directly
Preprocessor needs CPPFLAGS to find includes

On FreeBSD and OS X preprocessor cannot find libgsl. This patch will fix
./configure waring "accepted by the compiler, rejected by the preprocessor"
Improve checking for --enable-debug in configure.ac

- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for libpcap in configure.ac

- use AC_ARG_WITH instead of AC_ARG_ENABLE. BCP when checking for
  external library
- exit ./configure w/ an error if user request libpcap but library isn't
  found
- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for libgsl in configure.ac

- use AC_ARG_WITH instead of AC_ARG_ENABLE. BCP when checking for
  external library
- exit ./configure w/ an error if user request libgsl but library isn't
  found
- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for CURL in configure.ac

- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters