config-linux: Make linux.seccomp.syscalls OPTIONAL

Before this commit, linux.seccomp.sycalls was required, but we didn't
require an entry in the array.  That means '"syscalls": []' would be
technically valid, and I'm pretty sure that's not what we want.

If it makes sense to have a seccomp property that does not need
syscalls entries, then syscalls should be optional (which is what this
commit is doing).

If it does not makes sense to have an empty/unset syscalls then it
should be required and have a minimum length of one.

Before 652323c (improve seccomp format to be more expressive,
2017-01-13, #657), syscalls was omitempty (and therefore more
optional-feeling, although there was no real Markdown spec for seccomp
before 3ca5c6c, config-linux.md: fix seccomp, 2017-03-02, #706, so
it's hard to know).  This commit has gone with OPTIONAL, because a
seccomp config which only sets defaultAction seems potentially valid.

Signed-off-by: W. Trevor King <wking@tremily.us>

config-linux.md

@@ -610,7 +610,7 @@ The following parameters can be specified to setup seccomp:
     * `SCMP_ARCH_PARISC`
     * `SCMP_ARCH_PARISC64`
 
-* **`syscalls`** *(array of objects, REQUIRED)* - match a syscall in seccomp.
+* **`syscalls`** *(array of objects, OPTIONAL)* - match a syscall in seccomp.
 
     Each entry has the following structure:
 

specs-go/config.go

@@ -484,7 +484,7 @@ type WindowsNetworkResources struct {
 type LinuxSeccomp struct {
 	DefaultAction LinuxSeccompAction `json:"defaultAction"`
 	Architectures []Arch             `json:"architectures,omitempty"`
-	Syscalls      []LinuxSyscall     `json:"syscalls"`
+	Syscalls      []LinuxSyscall     `json:"syscalls,omitempty"`
 }
 
 // Arch used for additional architectures