treewide: fix typos

- mostly: usecase -> use case
- continously -> continuously
- single typos in docs/FILE_DESCRIPTOR_STORE.md

TODO

@@ -1053,7 +1053,7 @@ Features:
 * maybe add a generator that reads /proc/cmdline, looks for
   systemd.pull-raw-portable=, systemd-pull-raw-sysext= and similar switches
   that take a URL as parameter. It then generates service units for
-  systemd-pull calls that download these URLs if not installed yet. usecase:
+  systemd-pull calls that download these URLs if not installed yet. Use case:
   invoke a VM or nspawn container in a way it automatically deploys/runs these
   images as OS payloads. i.e. have a generic OS image you can point to any
   payload you like, which is then downloaded, securely verified and run.

docs/FILE_DESCRIPTOR_STORE.md

@@ -81,7 +81,7 @@ And that's already the gist of it.
 A system service that provides a client-facing interface that shall be able to
 seamlessly restart can make use of this in a scheme like the following:
 whenever a new connection comes in it uploads its fd immediately into its
-fdstore. At approporate times it also serializes its state into a memfd it
+fdstore. At appropriate times it also serializes its state into a memfd it
 uploads to the service manager — either whenever the state changed
 sufficiently, or simply right before it terminates. (The latter of course means
 that state only survives on *clean* restarts and abnormal termination implies the
@@ -104,7 +104,7 @@ lifecycle management (i.e. `KillMode=none` must be set), which disables large
 parts of the service managers state tracking, resource management (as resource
 counters cannot start at zero during service activation anymore, since the old
 processes remaining skew them), security policies (as processes with possibly
-out-of-date security policies – selinux, AppArmor, any LSM, seccomp, BPF — in
+out-of-date security policies – SElinux, AppArmor, any LSM, seccomp, BPF — in
 effect remain), and similar.
 
 # File Descriptor Store Lifecycle
@@ -176,7 +176,7 @@ This mechanism can be enabled either by making sure the service survives until
 the very end (i.e. by setting `DefaultDependencies=no` so that it keeps running
 for the whole system lifetime without being regularly deactivated at shutdown)
 or by setting `FileDescriptorStorePresever=yes` (and referencing the unit
-continously).
+continuously).
 
 # Debugging
 

src/basic/pidref.h

@@ -3,7 +3,7 @@
 
 #include "macro.h"
 
-/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continously. */
+/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continuously. */
 typedef struct PidRef {
         pid_t pid; /* always valid */
         int fd;    /* only valid if pidfd are available in the kernel, and we manage to get an fd */