use "curl -I" for header verification
Reduce fd usage, support convert_meta_tags

This reduced fd usage in NgxBaseFetch, and fixes support for convert_meta_tags

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/56

Squash merge of chaizhenhua's https://github.com/pagespeed/ngx_pagespeed/pull/241

Conflicts:
	src/ngx_pagespeed.cc
Reduce fd usage & support convert_meta_tags

This reduced fd usage in NgxBaseFetch, and fixes support for convert_meta_tags

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/56

Squash merge of chaizhenhua's https://github.com/pagespeed/ngx_pagespeed/pull/241

Conflicts:
	src/ngx_pagespeed.cc
thread-system: hold on to the NgxThreadSystem forPermitThreadStarting()
Merge commit '3927541c27c2376b543e046d9ed80bf6a162fc8c'
Merge pull request #281 from pagespeed/jefftk-thread-system

thread-system: hold on to the NgxThreadSystem for PermitThreadStarting()
correct a last minute error I made in the previous commit
Merge pull request #280 from pavel-paulau/patch-1

use "curl -I" for header verification
Merge pull request #282 from pagespeed/oschaaf-fix-compilation

correct a last minute error I made in the previous commit
system-test: convert meta tags isn't expected to fail anymore