With this change, it is now possible to use ngx_conf_merge_ptr_value()
to merge keyval arrays.  This change actually follows much earlier
changes in ngx_conf_merge_ptr_value() and ngx_conf_set_str_array_slot()
in 1452:cd586e963db0 (0.6.10) and 1701:40d004d95d88 (0.6.22).

To preserve compatibility with existing 3rd party modules, both NULL
and NGX_CONF_UNSET_PTR are accepted for now.

Previously, a chunk of spaces larger than NGX_CONF_BUFFER (4096 bytes)
resulted in the "too long parameter" error during parsing such a
configuration.  This was because the code only set start and start_line
on non-whitespace characters, and hence adjacent whitespace characters
were preserved when reading additional data from the configuration file.
Fix is to always move start and start_line if the last character was
a space.

Specifically, it is now allowed to start with a variable expression with braces:
${name}.  The opening curly bracket in such a token was previously considered
the start of a new block.  Variables located anywhere else in a token worked
fine: foo${name}.

Files are considered the same if the path used by nginx during parsing matches.

All the errors that prevent loading configuration must be printed on the "emerg"
log level.  Previously, nginx might silently fail to load configuration in some
cases as the default log level is "error".