Clarify the usage of callback fields inside oneof sections in the docs

cmake: Include CMAKE_INSTALL_LIBDIR in CMAKE_INSTALL_CMAKEDIR

cmake: Allow to build a shared library

If user defines non-standard directory in CMAKE_INSTALL_LIBDIR
option (i.e. /usr/lib64), then CMAKE_INSTALL_CMAKEDIR should
use it as a prefix.

Introduce options BUILD_SHARED_LIBS and BUILD_STATIC_LIBS to allow
building both types of libraries. By default only BUILD_STATIC_LIBS
is enabled.

Made some changes for Error Output.

Sometimes python protobuf version can't be found out by the above listed command. So added one more command to find python protobuf version which utilises pip freeze command.

Update README.md w/minor formatting

Add minor formatting marks to make more readable. Ex: `` for `code` and ** for *italics*.

pb_decode double free error with garbage input, PB_ENABLE_MALLOC and callbacks

If submessage array is a callback, there's nothing to release.

This bug can only occur when all of the following apply:

1) Message contains extension fields and
2) the extension field is of callback type and
3) the pb_extension_t structure is defined, but the pointer to
   pb_callback_t is left as NULL.

The correct behaviour in this case is that the field will be skipped.

Further, for most compilers atleast and probably for all compilers, the
bug will only occur if PB_OLD_CALLBACK_STYLE compilation option is specified.

Previously the generator assumed that any submessage could be found
in a different file. Now it knows that if the submessage is in current file,
it won't be in a different file.

Instead of 2**n ternary operator construct, uses a sizeof(union{})
construct with linear length increase.

Pass protoc-gen-nanopb.bat to protoc when on Windows

Feature: Flattening nested protobuf names

Three options for what to do with long type names:

M_NONE - original, default. Do nothing special.

M_STRIP_PACKAGE - Strip local package name. Useful when you want to use
package names, but don't want to deal with
`com_mycompany_supersoftware_proto_` prefixes on every symbol.
Not useful when your packages cross-reference each other, because it only
strips the _current_ package name, so references to other packages
actually become invalid.

M_FLATTEN - Always use only the last part of a dotted name.
Useful when you have packages and/or deeply nested structs that don't
actually have colliding names. Then you get shorter C symbols for no
additional cost.
Not useful when the names can collide.

Cmake relpath fix