Add fs_nvram_verify.
Reimplement nvram_export.

Closes issue #16
Rename vm_instance_t.ios_config to ios_startup_config.
Expand vm_platform_t.nvram_push_config to receive both startup-config and private-config.
Expand vm_platform_t.nvram_extract_config to get both startup-config and private-config.
Add vm_instance_t.ios_private_config.
Rework m_read_file to read the whole file at once.
Rework vm_nvram_push_config to handle both config files (NULL to keep existing data).
Extend hypervisor command 'set_config':
- new optional argument for the file with private data.
- new behaviour: an empty string "" means no config file.
Rework vm_ios_set_config to handle both config files (NULL to keep existing data).
Update documentation.

Related to issue #15.
Extend hypervisor command 'extract_config':
- return both startup-config and private-config data.
- empty data is sent as an empty string ''.
Update documentation.

Related to issue #15.
Extend hypervisor command 'push_config':
- new optional argument for the private data.
- new behavior: the string '(keep)' means no data is provided and it should preserve existing data.
Update documentation.

Related to issue #15.
Add command line options --private-config and --startup-config.
Update documentation.

Closes issue #15.
Fix warning "dereferencing type-punned pointer will break strict-aliasing rules".
Fix m32_tx_acquire not checking the HOLD bit properly.
Fix warning "suggest parentheses around operand of ‘!’ or change ‘&’ to ‘&&’ or ‘!’ to ‘~’".

Note: the datasheet isn't available anymore, so the intended behavior was inferred from surrounding code; it was assumed that the hold bit is managed elsewhere
Fix warnings "assignment discards ‘const’ qualifier from pointer target type" by copying the backplane/supervisor eeproms.
Fix memory leaks of c6sup1_t.slot_eeprom data.
Fix warnings "‘<function name>’ defined but not used".

__unused for function that are not referenced
__maybe_unused for functions that are referenced in excluded code
Fix warnings "label ‘<name>’ defined but not used".
Fix warnings "unused variable ‘<name>’".

UNUSED() for variables that are not used
__maybe_used for variables that are used in excluded code
Fix warnings "variable ‘<name>’ set but not used".

delete unneeded variables that don't help understand the surrounding code
__maybe_unused for the other variables
Adjust Makefiles:
- clean nvram_export.o
- add missing $(BIN_EXT)
- add target 'both' to facilitate test compilations
Bump version to v0.2.10.
Add release tags to ChangeLog.
Bump version in makefiles to v0.2.11-dev.