Update textproc/libxml2 to 2.9.6.

Update Portability:
- Change preprocessor OS tests to __linux__

Bug Fixes:
- Fix XPath stack frame logic
- Report undefined XPath variable error message
- Fix regression with librsvg
- Handle more invalid entity values in recovery mode
- Fix structured validation errors
- Fix memory leak in LZMA decompressor
- Set memory limit for LZMA decompression
- Handle illegal entity values in recovery mode
- Fix debug dump of streaming XPath expressions
- Fix memory leak in nanoftp
- Fix memory leaks in SAX1 parser

(fhajny)