Support legacy usage of SYS_mknod and SYS_mknodat

[jxyang]

According to Linux man page:

POSIX.1-2001 says: "The only portable use of mknod() is to create a
FIFO-special file. If mode is not S_IFIFO or dev is not 0, the
behavior of mknod() is unspecified." However, nowadays one should
never use mknod() for this purpose; one should use mkfifo(3), a
function especially defined for this purpose.

However, we don't support mkfifo in Myst either. And it seems some applications, such as dotnet runtime, still uses mknod to create named pipes.

[paulcallen]

currently we only support this for unix domain sockets, but not for pipes devices.

[bodzhang]

Current Mystikos implementation returns success on mknod(S_IFIFO), without creating a FIFO-special file.

[mikbras]

.NET attempts a mknod system call unless the following environment variable is define.

COMPlus_EnableDiagnostics=0

[vtikoo]

For tracing related dotnet tests, COMPlus_EnableDiagnostics needs to be turned on. One of the threads keeps reading on a pipe created via mknod in a loop. This fails currently as the /tmp path related to the pipe is not present in the enclave rootfs, causing an indefinite loop.