Add NVRAM filesystem.

Can read startup-config in raw format and .Z compressed format.
Can read private-config in raw format.
Can write startup-config and private-config in raw format.
Add generic_nvram_extract_config and generic_nvram_push_config that use the NVRAM filesystem.
Reimplement the nvram_extract_config/nvram_push_config functions of the platforms c1700/c2600/c2692/c3600/c3725/c3745/c7200/c6msfc1.
Fix the file handle leak in c6sup1_nvram_extract_config.

NOTE - the c6sup1 platform didn't produce a NVRAM filesystem on the test run, so I left it as is

Fixes issue #11 - nvram:private-config is destroyed when we write nvram:startup-config
Fixes issue #13 - C3745 has an emulated NVRAM that is not continuous, so we can generate an invalid checksum when the config is written
Fixes issue #14 - a file descriptor is leaked every time the config is extracted/read
Delete dev_vtty.c.orig.