This patch includes packaging changes related to files move.

Name and value in each header are 0-terminated, so additional 2 bytes
should be allocated for them.  There were several attempts to add these
2 bytes to headers in language modules, but some modules weren't updated.
Also, adding these 2 bytes is specific to the implementation which may be
changed later, so extending this mechanics to modules may cause errors.

Header names and values are stored 0-terminated for ease of use in different
languages, so magic number 2 should be added to each name-value pair size.

One small step to Go modules support.

To pass Go object references to C and back we use hack with casting to
unsafe and then to uintptr.  However, we should not store such references
because Go not guaratnee it will be available by the same address.
Introducing map with integer key helps to avoid dereference stored address.

This closes #253 and #309 issues on GitHub.

This closes #223 issue on GitHub.

Previously, the nxt_router_prepare_msg() function expected server host among
other headers unmodified.  It's not true anymore since normalization of the
Host header has been introduced in 77aad2c142a0.

The nxt_unit_split_host() function was removed.  It didn't work correctly with
IPv6 literals.  Anyway, after 77aad2c142a0 the port splitting is done in router
while Host header processing.

Library now used in all language modules.
Old 'nxt_app_*' code removed.

See src/test/nxt_unit_app_test.c for usage sample.