libsel4: Turn seL4_MsgMaxLength into an enum.
libsel4: Turn seL4_MsgMaxLength into an enum.

This has no effect on the code, but makes this value visible as a named
constant in Isabelle when importing C sources. It avoids us having to reference
120 as a magic number in numerous proofs.

dailypips/seL4,sigma-random/seL4,watbe/seL4,sigma-random/seL4,dailypips/seL4,codeneomatrix/seL4,sigma-random/seL4,DavidVorick/seL4,zhicheng/seL4,DavidVorick/seL4,cmr/seL4,watbe/seL4,dailypips/seL4,gpbonillas/seL4,rzhw/seL4,rzhw/seL4,lukw00/seL4,codeneomatrix/seL4,sigma-random/seL4,gpbonillas/seL4,sutt0n/seL4,sutt0n/seL4,CarterTsai/seL4,dailypips/seL4,lukw00/seL4,CarterTsai/seL4,rzhw/seL4,gpbonillas/seL4,CarterTsai/seL4,haitao52198/seL4,winksaville/seL4,DavidVorick/seL4,sutt0n/seL4,zhicheng/seL4,watbe/seL4,haitao52198/seL4,winksaville/seL4,cmr/seL4,lukw00/seL4,haitao52198/seL4,haitao52198/seL4,sigma-random/seL4,CarterTsai/seL4,lukw00/seL4,DavidVorick/seL4,cmr/seL4,gpbonillas/seL4,zhicheng/seL4,sutt0n/seL4,watbe/seL4,codeneomatrix/seL4,winksaville/seL4,rzhw/seL4,winksaville/seL4,codeneomatrix/seL4