Kernel: Use FixedStringBuffer for fixed-length strings in syscalls

Using the kernel stack is preferable, especially when the examined
strings should be limited to a reasonable length.

This is a small improvement, because if we don't actually move these
strings then we don't need to own heap allocations for them during the
syscall handler function scope.

In addition to that, some kernel strings are known to be limited, like
the hostname string, for these strings we also can use FixedStringBuffer
to store and copy to and from these buffers, without using any heap
allocations at all.

Base/usr/share/man/man2/pledge.md

@@ -65,6 +65,7 @@ Promises marked with an asterisk (\*) are SerenityOS specific extensions not sup
 * `EFAULT`: `promises` and/or `execpromises` are not null and not in readable memory.
 * `EINVAL`: One or more invalid promises were specified.
 * `EPERM`: An attempt to increase capabilities was rejected.
+* `E2BIG`: `promises` string or `execpromises `string are longer than all known promises strings together.
 
 ## History
 

Base/usr/share/man/man2/unveil.md

@@ -68,6 +68,7 @@ the error.
 * `EPERM`: The veil is locked, or an attempt to add more permissions for an
   already unveiled path was rejected.
 * `EINVAL`: `path` is not an absolute path, or `permissions` are malformed.
+* `E2BIG`: `permissions` string is longer than 5 characters.
 
 All of the usual path resolution errors may also occur.