Fixed REF_PATH to handle URLs better.

Without a REF_PATH it now defaults to using the EBI's MD5 service.
With a REF_PATH it now parses it in a friendly fashion. The URL= part
is now optional, instead auto-detecting http: and ftp: (no https yet).
Also even when using : as a path separator, when faced with a URL it
knows the syntax and treats the colons in http://host:80 as part of
the URL and not the separator. The old :: escape still works.
Suppress compression suffix search on default EBI MD5 service

The default REF_PATH setting should not query EBI's MD5 service for
compressed files like <md5>.Z, because such queries will always fail.

Also fix the URL colon auto-detection when used in conjunction with '|'
to suppress suffix searching.
Merge REF_PATH fixes and libcurl removal (PR #111)

Eventually we will want an htslib-native open_path_hfile() and then to
get rid of open_trace_file.[ch] and associated code, but for now this
implementation serves us well.

(Also fix Makefile dependencies and use hclose_abruptly() for cleanup.)